The present invention relates to the field of integrated circuits, and is more specifically directed to voltage regulators. This application is a reissue of 08/574,609 filed on Dec. 13, 1995, now U.S. Pat. No. 5,744,944.
In order to reduce power consumption many electronic components are now capable of operating at several power consumption levels, one of which is typically a power-down or sleep mode. The power-down mode is a low power consumption level that the component can enter into when it is not performing an operation or being accessed. Reducing the level of power consumption is particularly useful for battery operated devices, such as portable computers, where reducing power consumption increases the battery life, and therefore the time the device can be used without having to either replace or recharge the battery.
A microprocessor consumes a significant amount of power in the full power-up mode, and it is typically accessed only a small portion of the time, therefore it is advantageous to bring the microprocessor into the power-down mode when it is not being accessed. In the power-up mode the microprocessor can draw a current of up to 10 A. In the power-down mode the microprocessor can maintain its state with a current of as little as 100 xcexcA (i.e. as much as 100,000 times less than in the power-up mode). A power supply, such as a battery, typically supplies the current to the microprocessor through a voltage regulator. The time in which the voltage regulator can go from delivering the proper voltage for the current required in the power-down mode to delivering the proper voltage for the current required in the power-up mode, and vis versa, is the transient response time of the voltage regulator. The large change in current demand of the microprocessor, which is the load of the voltage regulator, may bring the voltage regulator out of regulation during the transient response.
The stability of the voltage regulator is primarily dependent on the size of its compensation capacitor, its load capacitor, and its pass element. The compensation capacitor is the capacitor put in at the gain stage of the voltage regulator to compensate the phase shift of the voltage regulator, and hence prevent oscillation. A larger compensation capacitor increases the stability of the voltage regulator. The size of this capacitor is inversely proportional to the bandwidth of the voltage regulator, and directly proportional to the transient response time of the voltage regulator. The larger the compensation capacitor the more stable the voltage regulator, and, unfortunately, the smaller the bandwidth of the voltage regulator. The smaller bandwidth does not allow the voltage regulator to respond quickly to large changes in current demand, increasing the transient response time.
Additionally, since the stability of the voltage regulator is dependent on the load capacitor as well as the compensation capacitor, as the load capacitor is reduced the voltage regulator becomes less stable. A smaller load capacitor is both easier to use and easier to recycle after the circuit is no longer needed, however it reduces the stability of the voltage regulator. The decrease in the load capacitor can be compensated by an increase in the compensation capacitor to ensure the stability of the circuit. However, as discussed above, an increase in the compensation capacitor reduces the bandwidth of the voltage regulator, and therefore increases its transient response time.
In some configurations proposed circuits have changed the bandwidth of the voltage regulator when the feedback loop of the voltage regulator indicated that the voltage regulator is out of regulation because of the large change in current demand. Alternative proposals have suggested monitoring circuits that could monitor the current demand of the load and notify the voltage regulator after there is a change in the current demand of the load.
A problem with the above methods is that during the time that the change in current demand is going through the feedback loop, or by the time the monitoring circuit detects that the current demand of the load has changed, the load is not receiving the appropriate voltage. The present inventor has observed that another problem is that during the time it takes to detect the change in the current demand the load is not receiving the proper voltage due to the large current demand, the voltage regulator does not know that it is not delivering the required voltage and has not even changed its bandwidth, thus increasing the transient response time by this amount of time.
Further background on voltage regulators and on stability criteria of analog devices can be found in Grebene, Alan B., BIPOLAR AND MOS ANALOG INTEGRATED CIRCUIT DESIGN, John Wiley and Sons, 1984; Gray, Paul R. and Robert G. Meyer, ANALYSIS AND DESIGN OF ANALOG INTEGRATES CIRCUITS, 2nd ed., John Wiley and Sons, 1984; Franklin, Gene F. et al., FEEDBACK CONTROL OF DYNAMIC SYSTEMS, Addison-Wesley Publishing Co., 1986; SGS-THOMSON, Microelectronics, Inc. databooks for voltage regulators, for linear ICs, and for automotive products; and National Semiconductor databooks, datasheets and application notes for voltage regulators, all incorporated herein by this reference.
The present application discloses methods and circuits for reducing the transient response time of a voltage regulator when the load attached to it is entering or exiting a lower power consumption level, without compromising the stability of the voltage regulator. A control device is connected to both a voltage regulator and a dynamic load. When the control device senses that the dynamic load should change its power consumption level it sends a signal to a regulation means in the voltage regulator to change the bandwidth of the voltage regulator to be able to deliver the required voltage in the shortest possible time. At the same time (or shortly thereafter, depending on the requirements of the system) the control device sends a signal to the dynamic load to change its power consumption level. In one sample embodiment of the invention, after the voltage regulator starts delivering the required voltage the regulating means returns the bandwidth of the voltage regulator to the optimal bandwidth for the dynamic load connected to the voltage regulator.
In one embodiment of the invention the regulating means includes a way of switching the compensation capacitor in or out when the control device signals the voltage regulator to change the bandwidth of the voltage regulator. In another sample embodiment of the invention the regulating means includes a switched capacitor that changes the bandwidth of the voltage regulator when the current demand of the load is about to change.
One advantage of the present invention is that the bandwidth of a voltage regulator can be increased in order to reduce the voltage regulator""s transient response, without compromising the voltage regulator""s stability.
Another advantage of this invention is that there is no delay time from the time the current requirement of the dynamic load increases until the time the bandwidth of the voltage regulator increases, to allow the voltage regulator to deliver the required voltage faster.
A further advantage of this invention is that it allows a load to enter the power-down mode faster, reducing the power consumed by the load.
Another advantage of this invention is that it allows the use of a smaller load capacitor without compromising the stability of the voltage regulator.
A particularly advantageous use of the current invention is in battery-powered, portable computers. The improved transient response time of the voltage regulator allows aggressive power-conservation strategies in which the microprocessor frequently enters sleep mode.
Other advantages and objects of the invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.